1. Field of the Invention
The present invention relates to an inverter which supplies a driving voltage to a fluorescent lamp etc. and, more particularly, to an inverter provided with the dimming function for adjusting the brightness of a fluorescent lamp.
2. Description of the Related Art
In recent years, liquid crystal televisions have been extensively used in place of cathode-ray tube televisions, due to the availability of lightweight or thin LCDs. A liquid crystal television is configured such that multiple cold cathode fluorescent lams (hereinafter, referred to as CCFLs) or external electrode fluorescent lamps (hereinafter, referred to as EEFLs) are placed behind a liquid crystal panel on which images are displayed, so as to provide a backlight.
For example, an inverter (dc/ac converter) which boosts a dc voltage of about 12V and outputs an ac voltage is used to drive a CCFL or an EEFL. An inverter converts a current which flows in the fluorescent lamp into a voltage, feeds back the voltage to a control circuit, and controls on/off of a switching element in accordance with the voltage thus fed back. For example, the patent document No. 1 discloses such a technology for driving a fluorescent lamp by an inverter.
The control circuit for an inverter may be provided with a dimming function in order to perform dimming control whereby the brightness of a fluorescent lamp is adjusted. There are two kinds of dimming control. One is set up by a set manufacturer designing equipment equipped with a fluorescent lamp and an inverter, and the other is set up by a user using the equipment. Methods for light control include analog dimming control whereby a current flowing in a fluorescent lamp is controlled (current-based dimming control) and burst dimming control whereby a fluorescent lamp is turned on intermittently (see, for example, the patent document No. 2).
[Patent document No. 1 ]
JP 2003-323994
[Patent document No. 2 ]
JP 2005-108784
We have come to be aware of the following challenge faced in designing an inverter control circuit capable of performing analog dimming control and burst dimming control at the same time.
Analog dimming control or burst dimming control requires the use of a periodic signal such as a triangular wave. When performing analog dimming control and burst dimming control at different frequencies, a periodic signal in one circuit block may affect the other. For example, when the frequency of analog dimming control is set on the order of 10-100 kHz and the frequency of burst dimming control is set on the order of 10-100 Hz, circuit operation may become unstable as a result of the periodic signal for analog dimming control being introduced into the circuit block for burst dimming. The above consideration shall not be construed as a concession of the bound of general knowledge applicable to the field of the invention.